Crystal structure of barium dinickel(II) iron(III) tris[orthophosphate(V)], BaNi2Fe(PO4)3

The orthophosphate BaNi2Fe(PO4)3 crystallizes in the α-CrPO4 type of structure, in which edge-sharing [Ni2O10] octahedra are linked to PO4 tetrahedra and [FeO6] octahedra to form a three-dimensional framework delimiting channels which house disordered Ba2+ cations.


Chemical context
Phosphate-based materials have been studied extensively in the past. Among them are orthophosphates, which have gained great interest in recent years owing to their structural richness (Maeda, 2004) and their promising applications, for example in electrochemical catalysis (Dwibedi et al., 2020;Cheng et al., 2021;Rekha et al., 2021;Anahmadi et al., 2022). Furthermore, orthophosphates doped with rare-earth cations have shown excellent optical properties (Ci et al., 2014;Li et al., 2021;Indumathi et al., 2022), along with a wide range of applications for use in luminescence emission displays (Li et al., 2008;Wan et al., 2010;Yang et al., 2019;Santos et al., 2022).
In this context, our research interest is connected with trisorthophosphate-based materials with general formula (A 2 / B)M 2 M 0 (PO 4 ) 3 , where A can be an alkali, B an alkaline earth and M and M 0 transition metal cations. The crystal structures of these orthophosphates adopt the -CrPO 4 type of structure, consisting of a three-dimensional framework made up of [MO 6 ] and [M 0 O 6 ] octahedra sharing corners and/or edges with PO 4 tetrahedra. This framework is permeated by channels in which the A or B cations are located.
To confirm the structure model of BaNi 2 Fe(PO 4 ) 3 , the bond-valence method (Brown, 1977;1978;Brown & Altermatt, 1985) and charge distribution (CHARDI) concept (Hoppe et al., 1989) were employed by making use of the programs EXPO2014 (Altomare et al., 2013) and CHARDI2015 (Nespolo & Guillot, 2016), respectively. Table 1  The principal building units in the crystal structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
Projection of a (100)    factors sof(i)]. In summary, the expected oxidation states of Ba 2+ , Ni 2+ , Fe 3+ and P 5+ are predicted through the charge distribution. The internal criterion q(i)/Q(i) is very near to 1 for all ionic species and the mean absolute percentage deviation (MAPD), which gives a measure for the agreement between the q(i) and Q(i) charges, is just 1.3%, thus confirming the validity of the structure model (Eon & Nespolo, 2015). The global instability index (GII; Salinas-Sanchez et al., 1992) of 0.13 is a further confirmation of the structure model.

Database survey
It is reasonable to compare the crystal structure of the title compound with that of -CrPO 4 (Glaum et al., 1986). Both phosphates crystallize in the orthorhombic system in space group type Imma. Their unit-cell parameters are nearly the same despite the differences between their chemical formulae.
In the structure of -CrPO 4 , the Cr 3+ and P 5+ cations occupy four special positions that are part of a framework is comprised of [

Synthesis and crystallization
BaNi 2 Fe(PO 4 ) 3 was prepared from a mixture of Ba(NO 3 ) 2 (Merck, 98.5%), Ni(NO 3 ) 2 Á6H 2 O (Riedel-de-Haé n, 97%), Fe(NO 3 ) 3 Á9H 2 O (Panreac, 98%) and H 3 PO 4 (85% wt ) in the molar ratio of Ba:Ni:Fe:P = 1:2:1:3. The precursors were suspended in 50 ml of distilled water and stirred without warming for 24 h before heating to dryness at 373 K. The obtained dry residue was ground in an agate mortar until homogeneous, subsequently heated in a platinum crucible up to 673 K to remove volatile decomposition products, and then melted at 1433 K. After being kept at this temperature for one h, the melt was cooled down slowly at a rate of 5 K h À1 to 1233 K and then to room temperature. Single crystals with a brown color and different forms were obtained after leaching with distilled water. Chemical analysis of the title phosphate was performed with an energy-dispersive X-ray spectroscopy (EDS) microprobe mounted on a JEOL JSM-IT100 in TouchScope TM scanning electron microscope. The EDS spectrum is depicted in Fig. 6 and confirms the presence of only barium, nickel, iron, phosphorus and oxygen in approximately the correct ratios, as shown in Table 2.

Figure 5
Polyhedral representation of the crystal structure of BaNi 2 Fe(PO 4 ) 3 showing Ba1 and Ba2 in the channels. Table 1 Bond valence and CHARDI analyses for the cations in the title compound.

Barium dinickel(II) iron(III) tris[orthophosphate(V)]
Crystal data Extinction correction: SHELXL2018/3 (Sheldrick, 2015b), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.00403 (16) Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )